This mod relies on a simple side
branch resonatorand WalkerQuiet Flow muffler,
both
shown above.

Kolak's
exhausts are generally well
regarded within the Jeep performance community. Unfortunately I was
disappointed
with the setup provided
for my '99 JGC Laredo ( 4.7L V8,
Quadra-Drive,
up
country, tow package). I found it was too loud and drone
prone for my
needs. (What can I say, I guess I'm old!)

Fortunately, after a lot of
homework
and three followup attempts,
I'm happy to say I finally have a configuration that fixed the
noise issue
to my satisfaction.

Here are the components I am running:

•
MagnaFlow 94409 catalytic converter / resonator• 2.25"
x 30" custom side branch resonator• Walker
Quiet Flow 21054 muffler• Kolak
3" mandrel bent tail pipeNote.
This is not intended to be a one
size fits all recommendation. Consider it to be more general
guidance. Details may vary according to circumstances.
My suggestion would be to first install the Quiet Flow muffler, then
check for
drone, and finally add a resonator tuned to eliminate any actual
drone. An alternative, if you're happy with the overall
sound of
your exhaust and just want to cure drone, would be to
try to
figure out
a way to fit a resonator while retaining the existing muffler.

My
Experience.When I first contacted Kolak
about an exhaust I
explained that I wasn't young anymore and I did not want a loud system. Kolak
recommended that I
go with a
"stealthier" MagnaFlow muffler rather than one of the Flowmasters he
typically
supplies.
He provided a system consisting of his custom tail
pipe, a MagnaFlow
12289 24" case muffler and MagnaFlow 94009 high flow
catalytic
converter.

I still found this
system to
be too loud, particularly the drone at expressway cruising
speeds (1,700 - 2,200 RPM). There was also cabin interior
resonance (a "beating" sensation I could feel on my ear drums)
around 1,600-1,700 RPM. Kolak's response was that most customers
are happy with
the sound level of his systems, but that sometimes variations in the
installation
might cause drone. He also mentioned that the
early 49-state-emission
WJs
without pre-cats do tend to be louder. In an attempt to address my
issue he replaced the catalytic
converter with a larger MagnaFlow model 94409 which includes a
resonator. The new converter did quiet the system somewhat but there
was still annoying drone at highway cruising speeds.

Still
dissatisfied, I had the system completely reinstalled by
a different exhaust shop, one with a top-notch reputation for custom
work. My hope was that a better
installation might correct any
other
issues contributing to the drone. I also had the MagnaFlow
muffler
swapped for a Walker Quiet Flow model 21054 - the
quietest muffler I
could
find which would fit and still provide good flow with the 3 inch pipes.
This
system
was significantly quieter overall but there was
still obnoxious
drone, though now it was at 1,500 -
2,000 RPM. These changes had moved the drone downward about
200 RPM, trading some of the cruising drone for still
annoying
around-town drone.

Finally,
after
spending a lot more time researching the cause and possible
cures of exhaust
drone, I had my shop
fabricate and install
a side
branch resonator. I tried to tune the resonator length
close to the center of my drone
band, but leaning a little toward the upper RPM.

I
was really pleased when the new resonator actually eliminated nearly
all drone. It seemed almost
magical. Only a slight drone remains right at 1,500 RPM. Perhaps that
could
also have been eliminated if the resonator length had
been tuned to work
best at
slightly
lower RPM, but when I was considering the tuning
I wanted to
be certain to cover all cruising RPM drone and I wasn't sure
whether one resonator could cover the entire range from 1,500 to 2,000
RPM. In view of this choice
I am really
happy it worked as well as it did over almost the entire band.

The
exhaust is
now far
quieter than before. The overall sound is
comparable to a
fairly subtle original equipment high performance
or heavy duty truck exhaust. It's not as
deep, throaty
and loud
as typical aftermarket performance systems, but there is considerably
more V8
character than with the stock exhaust.

Most
side branch resonator installations I've seen on the internet have been
mounted on the
tailpipe. However there is limited space under
this vehicle, especially
with a
24" muffler attached directly to the tailpipe, so the resonator had to
be attached in
front of the
muffler and routed along the inboard side between the muffler
and driveshaft. There is enough room there because the Quiet
Flow muffler is relatively narrow and also has an
offset
inlet and center
outlet (opposite of the Kolak and original equipment setup). That
configuration shifts
the casing away from the driveshaft providing more room on the
inboard side. It might be
possible to install a side branch with a wider muffler mounted
in the stock position, but that would
likely
require some clever pipe bending to fit in the
limited available
space.

One
additional consideration with placing the resonator in front of the
muffler is
that the temperature there is probably a little higher than it would be
at the tailpipe.
This would increase the speed of sound within
the resonator. Which means the
tuning length needs to be a bit longer to function at the
intended
frequency. Based on some later calculations, it
appears that
~½" to 1" longer tuning might possibly have worked better for hitting
the center of my drone band. This assumes a resonator
temperature
of 165° F rather than the 150° estimate I initially worked with.

Below
you'll find charts, formulas and links to
calculators other people
have provided to help determine the correct tuning length. These
tools provide only estimates. Perfect tuning requires either luck
or trial and error. One
trial and error approach would be a
temporary clamped-on resonator pipe that could easily be
resized, and then replaced with welded
pipe once the optimal tuning was determined. Fortunately tuning
doesn't have to be perfect to do a decent job. I
haven't seen one report of anyone trying this with estimated
tuning and not realizing improvement.

Drone.
It's easy to find information,
misinformation and speculation
about the cause of exhaust drone on the internet. After a lot of
reading I concluded that the drone in my system was caused by
resonance of exhaust
gas within the tailpipe. This occurs when the tailpipe
is effectively tuned to resonate at a certain frequency, similar to a
pipe in an organ. The tuning is determined primarily by the
length
of the
pipe. Whenever
the engine speed reaches the RPM which produces the "tuned"
frequency a resonance occurs within the pipe exaggerating that
sound.

The actual
resonant
frequency
can change somewhat depending on the temperature and composition of the
exhaust gas. The sound level will also depend on
the exhaust
flow (a function of throttle opening in addition to RPM). On
some vehicles the
resonance may be amplified by sympathetic
vibration of
the exhaust pipe. Whether this happens can depend on the mass
of the pipe, the position and stiffness of its mounts and
again on temperature.
In some cases the floor and/or interior of a vehicle can also
resonate at the
same frequency making the problem even worse. I think those
other
factors
are usually secondary. I
believe controlling
the resonance of the
exhaust gas within the pipe is the most important factor in
curing drone.

I
have
a further theory that the drone
frequency may actually be determined
not simply by the length of the tailpipe alone but also the length of
the final
muffler chamber (or chambers) in
combination
with the tailpipe.
Connected
together I believe they may form one (or more) longer effective
chamber(s). This would
explain
why the frequency of my drone was lowered when switching from the
MagnaFlow to
the Quiet Flow muffler. I haven't been able to verify this, but I
believe the
final chamber in the Quiet Flow may be longer than the one in the
MagnaFlow. This theory may also explain why my drone band is
so wide, i.e.,
potentially there are
effectively
two or three of these combined "chambers",
each
resonating at a slightly different frequency.

Side
Branch Resonator.
A side branch resonator is a simple approach for eliminating low
frequency exhaust
drone. (This principle is used on
a few original equipment exhaust systems, very large stationary engines, and also on engine
intake
systems.)

Capped pipe “T”ed into
exhaust.

Length determines tuning for target
sound frequency.

Sound wave for tuned frequency reflects
back 180° out of phase and cancels that frequency from exhaust.

The
tuned length is equal to 1/4 wavelength of the target frequency. This
means the reflected wave is 1/2 wavelength out of phase with the
primary exhaust wave which results incancellation. Seems like
magic but it’s actually physics.

There
is some approximation involved with the tuning, since it depends on
the
speed of sound
within the resonator and that is dependent on the composition
of the exhaust gas and operating
temperature, which can usually only be estimated.
The resonator tube size does not need to be the full diameter of the
main
exhaust
pipe. There is some
reduction in effectiveness using a somewhat smaller diameter tube for
the
resonator but it will still remove most of the drone.

With Different
Converter or Muffler.
I listed the exact catalytic converter and muffler I am using because I
am very satisfied with my current setup. Many people have successfully
applied
side branch resonators to cure drone using a variety
of exhaust components. So
there is no reason you could not do the same with the smaller
(louder)
94009 converter that Kolak usually supplies. You would simply have to
tune the resonator for the actual drone occurring with that setup. It
could also
work with a different muffler, but finding space to install the
resonator
might be a challenge.

Credit. Primary
credit for sharing this concept should go to Graham at
the Mustang corral.net forum
who pioneered and wrote about it back in 2004. Lots of others
also
deserve credit for sharing their experience. All I've done is put all
the useful
information I've found together on one page. Also credit to Keith's Muffler and Brakes in
Westland, Michigan for the fabrication and installation.

*Well
not
"exactly". I'm afraid the guy who wrote what I'm quoting is guilty
of false
precision.
He's using really precise values (lots of decimal
places), even though the margin of error may actually be ±5% due to
other factors. I think the length calculated with his formula also may
be a
bit too long, but is likely more accurate than the simple chart at the
top of these notes. The perfect tuning length depends upon the actual
gas
temperature
and composition, which we don't know for certain. So the
charts
and
formulas above
are only estimates of what might
work best. They should
be pretty close though. The calculators linked to below
may help
provide better estimates if you have better data.

Additional note in response to reader questions. For 4 and 6 cylinder engines the math relating drone frequency and resonator length to RPM is different than for eights.
Since there are fewer ignition firings per revolution with fours and
sixes compared to an 8 cylinder, the frequency of the primary exhaust
note at any given RPM will be lower. (i.e., the wave length is longer.)
This means that the resonator also needs to longer to cancel drone
at that RPM. For a four cylinder it should be twice as long. For a six
1.5 times as long.

Finally. It took
some time to put this write-up together, but nowhere near as
much as it took trying to get an exhaust I could live with.
Without members of
a couple of internet forums sharing their successes on other
vehicles, it might not have happened at all.

So this is for any other Jeep Grand Cherokee owners and others out there who might
be in the same
situation.